1. Field of the Invention
The present invention relates to SMIF pods, and more particularly to a SMIF pod shell enclosing an independently supported cassette allowing precise, controllable and repeatable positioning of wafers with respect to a surface on which the pod is supported.
2. Description of Related Art
A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers, and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafers and/or wafer cassettes; (2) an input/output (I/O) minienvironment located on a semiconductor processing tool to provide a miniature clean space (upon being filled with clean air) in which exposed wafers and/or wafer cassettes may be transferred to and from the interior of the processing tool; and (3) an interface for transferring the wafers and/or wafer cassettes between the SMIF pods and the SMIF minienvironment without exposure of the wafers or cassettes to particulates. Further details of one proposed SMIF system are described in the paper entitled xe2x80x9cSMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING,xe2x80x9d by Mihir Paikh and Ulrich Kaempf, Solid State Technology, July 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 microns (xcexcm) to above 200 xcexcm. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half xcexcm and under. Unwanted contamination particles which have geometries measuring greater than 0.1 xcexcm substantially interfere with 1 xcexcm geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.1 xcexcm and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles and molecular contaminants become of interest.
In practice, a SMIF pod is set down on various support surfaces within a wafer fab, such as for example at a load port to a minienvironment, whereupon interface mechanisms in the load port open the pod door to allow access to the wafers within the pod. Additionally, a pod may be supported at a storage location while awaiting processing at a particular tool. Such storage locations may comprise a local tool buffer in the case of metrology or high throughput tools, or may alternatively comprise a stocker for storing large numbers of pods within a tool bay. A pod may additionally be positioned at a stand-alone purge station.
Whether a tool load port, local tool buffer, stocker or purge station, the support surfaces typically include registration or kinematic pins protruding upward from the support surface. In 300 mm pods, a bottom surface of the pods includes radially extending grooves for receiving kinematic pins. Once the pod is positioned so that the grooves engage their respective kinematic pins, the grooves settle over the pins to establish six points of contact between the pod and support platform (at the grooves and pins) to kinematically couple the pod to the support platform with fixed and repeatable accuracy. Such a kinematic coupling is for example disclosed in U.S. Pat. No. 5,683,118, entitled xe2x80x9cKinematic Coupling Fluid Couplings and Methodxe2x80x9d, to Slocum, which patent is incorporated by reference herein in its entirety. The size and location of the kinematic pins are standardized so that the pods of various suppliers are compatible with each other. The industry standard for the location and dimensions of the kinematic coupling pins are set by Semiconductor Equipment and Materials International (xe2x80x9cSEMIxe2x80x9d).
In general, wafers may be supported within a pod according to one of two configurations. In a first configuration, the wafers may be seated within a removable cassette including a plurality of shelves for supporting the wafers in a planar orientation. The cassette in general includes kinematic pins or grooves on its bottom surface for mating with respective kinematic grooves or pins provided on an upper surface of the bottom of the pod. Thus, in the first configuration, wafers are supported by the wafer cassette, which is in turn supported within the pod, which is in turn supported on a support surface. The second configuration for supporting wafers within a pod is the so-called cassetteless pod. Such pods are used exclusively for front opening applications, and include a plurality of shelves formed on the side walls of the pod itself for supporting the wafers in a planar orientation. An example of such a pod is disclosed in U.S. Pat. No. 5,476,176 to Gregerson entitled, xe2x80x9cReinforced Semiconductor Wafer Holderxe2x80x9d.
Pods are typically formed of plastics and various polymers such as for example polycarbonate. These materials allow the pods to be efficiently and inexpensively manufactured of a lightweight material which is easily transported, and are typically transparent to allow viewing of the wafers seated therein. While it is conceivable that pods may be manufactured from various metals, metal pods are in general disfavored within wafer fabs owing in part to their weight and potential for ionic contamination.
The desired material characteristics of the wafer cassettes for supporting the wafers are different than those of the pods. It is desirable that the wafer cassettes be more rigid, temperature and wear resistant than the pods, and that the wafer cassettes be static dissipative. For at least these reasons, the pods and wafer cassettes are typically formed of different materials. One preferred material from which the wafer cassettes are formed is polyetheretherkeytone, or xe2x80x9cPEEKxe2x80x9d. Owing in part to its weight, expense and lack of transparency, PEEK is in general not a good material for use in forming a pod.
Once the pods and wafer cassettes are independently formed, the pod shells and wafer supports are generally affixed together in front opening pods to thereby constrain the wafer cassette against movement with respect to the pod shell in all six degrees of movement. That is, the wafer support is prevented from translating along X, Y, and Z cartesian axes, and is prevented from rotating about the X, Y and Z cartesian axes, with respect to the pod shell. The rigidity of the pod shell is relied upon to stabilize and maintain the wafer support in a proper position.
However, conventional pod shells have proven somewhat ineffective in providing a precise, controllable and repeatable positioning of the wafer supports within the pods. One reason is that inherent stresses within the pod shell cause the pod shell to slightly warp or deform over time. Additionally, mechanisms are provided at support surfaces such as for example those at load ports for physically grasping and securing the pod in tight engagement with both the horizontal support surface and the vertical load port. Such grasping and engagement of the pod may further cause deformation of the pod shell. Further still, pods weigh on the order of about twenty pounds. When the pods are lifted from a handle mounted on a top of the pod, as they often are, the pod shells may elongate slightly in the vertical direction, pulling the sides of the pod shell inward. Deformation of the pod shell as a result of any of the above described conditions is communicated directly to the wafer support, which as described above is typically connected to the pod shell in front opening pods.
The deformation or warping of the pod shell can therefore adversely affect the positioning and control of the wafer supports and wafers with respect to each of the X, Y and Z axes, as well as the planarity of the wafers within the wafer support. Conventional process tools use a support surface on which a pod is seated as a reference plane. A wafer access tool for transferring wafers to and from the pod expects the wafers to be at a predetermined height above the reference plane. Any variation in the expected X,Y and/or Z position, or wafer planarity, of the wafers with respect to the support surface may adversely affect wafer access by the wafer access tool, and/or damage the wafers as a result of unexpected contact between the wafers and the wafer access tool or wafer supports.
It is therefore an advantage of the present invention to provide a system for precise positioning of wafers in a known, controllable and repeatable position with respect to a support surface.
It is a further advantage of the present invention to be able to affix a cassette directly on a support surface such as that at a load port, while simultaneously encasing the wafers within a SMIF pod to isolate the wafers from contaminants and/or particulates.
It is another advantage of the present invention to allow pod-positioning mechanisms at a load port to securely position a pod at a port without jeopardizing a precise, controllable and repeatable positioning of the wafers with respect to the load port.
It is a still further advantage of the present invention to provide a cassette which may be removed from a pod so that the pod and/or cassette may be cleaned, and/or so that cassette may be interchanged within a particular pod.
It is another advantage to provide a system for removing electrostatic charge from the wafers.
It is a further advantage of the present invention to provide a cassette having a modular construction in which the support structure components may be individually removed and replaced by components of the same or different configuration and/or material.
It is a still further advantage of the present invention to allow the pod shell to be formed with thin walls, thereby reducing the weight and manufacturing costs of the pod, while at the same time providing a rigidly controlled positioning of the wafers within the pod shell.
These and other advantages are provided by the present invention which in preferred embodiments relates to a SMIF pod including an independently supported cassette. The pod preferably includes a conveyor plate mounted on its bottom surface, which conveyor plate includes three kinematic grooves for establishing a kinematic coupling on kinematic pins of a support surface on which the pod is seated. The cassette may be provided in a variety of configurations, each of which being capable of supporting a plurality of wafers at a fixed, controllable and repeatable position with respect to a surface on which the pod is supported, substantially regardless of any warping or deformation of the pod shell.
In a preferred embodiment of the invention, the cassette comprises a pair of rigid support columns located at the sides of the pod, and a top plate extending between and connecting the support columns. The support columns preferably include a plurality of shelves, with a shelf from each column together defining a plane in which a single semiconductor wafer may be securely supported. In this embodiment, support columns are preferably attached directly to the conveyor plate, through the pod shell, at or near the kinematic couplings. The top plate further improves the rigidity of the cassette. The top plate may alternatively be omitted, leaving a wafer support structure comprised solely of the pair of support columns rigidly affixed to the conveyor plate.
In further embodiments of the present invention, the cassette may include a bottom plate having legs which connect directly to the conveyor plate through the pod shell. Connected directly to the conveyor plate, the elevation and horizontal planarity of the bottom plate may be precisely, controllably and repeatably maintained. A pair of wafer support columns as described above may be affixed to the bottom plate, and a top plate as described above may further be included.
In embodiments of the present invention not including a bottom plate, a bottom portion of the support columns are fit with threaded bores, preferably two such bores per support column. Screws, located at or near the location of the kinematic couplings, are provided up through the conveyor plate, through a hole in the pod shell, and into the threaded bores in support columns to secure the support columns to the conveyor plate, substantially independent of the pod shell. An O-ring seal may further be provided between the pod and a bottom of the support columns around the threaded bore, so that an air tight seal is provided to prevent particulates and/or contaminants from entering into the pod when the screws are tightened. In a preferred embodiment, each of the wafer support columns, the mounting screws, and the kinematic grooves are electrically static dissipative so that electrostatic charge in the wafers may be drawn away from the wafers through the support columns, mounting screws, kinematic grooves, and finally down through the kinematic pins. In an embodiment of the cassette including a bottom plate, the bottom plate may include a plurality of legs extending down from the bottom plate, which legs are fit with threaded bores for mounting the bottom plate to the conveyor plate via the mounting screws as described above.
In a further embodiment of the present invention, either the support columns or the bottom plate may include legs which extend down through a hole formed in the pod. A bottom surface of each such leg preferably includes a kinematic groove for mating with corresponding pins on a support surface of the pod so that the pins and grooves together form a kinematic coupling directly between the cassette and the support surface. A sleeve having an annular seal, such as for example an O-ring seal, may be affixed to each of the holes in the pod through which a cassette leg extends, so that the sleeves and O-ring together provide a tight seal between the pod and the support structure legs. This tight seal prevents contaminants and/or particulates from entering into the pod between the pod shell and the support structure legs.
An upper portion of the cassette may include an upwardly extending fin which fits between a pair of fins formed on and extending down from an interior top surface of the pod. The fin arrangement prevents any significant side-to-side movement of the support columns within the pod, as for example upon a shock to the pod. In embodiments of the invention including a top plate, the fin arrangement may be located approximately in the center of the top plate. In embodiments of the invention not including a top plate, the fin arrangement may be provided on one or both of the support columns, preferably at the rear of the support columns.
Even in embodiments of the cassette including the above-described fin arrangement, it is a feature of the various embodiments of the present invention that the cassette is not affixed to the top or sides of the pod shell. Thus, the cassette will support the wafers in a fixed, repeatable and controllable position, which is substantially unaffected by deformation of the pod shell.